Perl's internal data types
1.3
Maintainer: Dan Sugalski <dan@sidhe.org>
Class: Internals
PDD Number: 4
Version: 1.3
Status: Developing
Last Modified: 02 July 2001
PDD Format: 1
Language: English
- Version 1.3, 2 July 2001
-
- Version 1.2, 2 July 2001
-
- Version 1.1, 2 March 2001
-
- Version 1, 1 March 2001
-
- Version 1.3
-
Fixed some silly typos and dropped phrases.
Took all the underscores out of the field names.
- Version 1.2
-
The string header format has changed some to allow for type
tagging. The flags information for strings has changed as well.
- Version 1.1
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INT and NUM are now concepts rather than data structures, as making
them data structures was a Bad Idea.
- Version 1
-
None. First version
This PDD describes perl's known internal data types.
This PDD details the primitive datatypes that the perl core knows how
to deal with. These types are lower-level than what's presented to the
perl programmer.
Integer data types are generically referred to as INTs. INTs are
conceptual things, and there is no data structure that corresponds to them.
- Platform-native integer
-
These are whatever size native integer was chosen at perl
configuration time. The C-level typedef
IV and UV get you a
platform-native signed and unsigned integer respectively.
- Arbitrary precision integers
-
Big integers, or bigints, are arbitrary-length integer numbers. The
only limit to the number of digits in a bigint is the lesser of the
amount of memory available or the maximum value that can be
represented by a
UV. This will generally allow at least 4 billion
digits, which ought to be far more than enough for anyone.
The C structure that represents a bigint is:
struct bigint {
void *buffer;
UV length;
IV exponent;
UV flags;
}
The num_buffer pointer points to the buffer holding the actual
number, length is the length of the buffer, exponent is the base
10 exponent for the number (so 2e4532 doesn't take up much space), and
flags are some flags for the bigint.
Note:The flags and exponent fields may be generally unused, but are
in to make the base structure identical in size and field types to
other structures. They may be removed before the first release of perl
6.
Floating point data types are generically reffered to as NUMs. Like
INTs, NUMs are a conceptual things, not a real data structure.
- Platform native float
-
These are whatever size float was chosen when perl was configured. The
C level typedef
NV will get you one of these.
- Arbitrary precision decimal numbers
-
Arbitrary precision decimal numbers, or bignums, can have any number
of digits before and after the decimal point. They are represented by
the structure:
struct bignum {
void *buffer;
UV length;
IV exponent;
UV flags;
}
and yes, this looks identical to the bigint structure. This isn't
accidental. Upgrading a bigint to a bignum should be quick.
Perl has a single internal string form:
struct perl_string {
void *buffer;
UV allocated;
UV bytes;
UV flags;
UV characters;
UV encoding;
UV type;
UV unused;
}
The fields are:
- buffer
-
Pointer to the start of the string's data.
- allocated
-
How many bytes are allocated in the buffer.
- bytes
-
How many bytes are used in the buffer.
- flags
-
Flags indicating whatever. Bits 0-15 are reserved for perl, bits 16-23
for the encoding/decoding code, and teh rest for the type code.
- characters
-
How many characters are in the buffer. An optional cache field.
- encoding
-
How the data is encoded, for example fixed 8-bit characters, utf-8, or
utf-32. An index into the encoding/decoding function table. Note that
this specifies encoding only--it's valid to encode EBCDIC characters
with the utf-8 algorithm. Silly, but valid.
- type
-
What sort of string data is in the buffer, for example ASCII, EBCDIC,
or Unicode. Used to index into the table of string functions.
- unused
-
Filler. Here to make sure we're both exactly double the size of a
bigint/bigfloat header and to make sure we don't cross cache lines on
any modern processor.
None
The perl modules Math::BigInt and Math::BigFloat. The Unicode standard
at http://www.unicode.org.
- Type
-
Type refers to a low-level perl data type, such as a string or integer.
- Class
-
Class refers to a higher-level piece of perl data. Each class has its
own vtable, which is a class' distinguishing mark. Classes live one
step below the perl source level, and should not be confused with perl
packages.
- Package
-
A package is a perl source level construct.